(1) Field of the Invention
The present invention relates to a process for the preparation of high-performance grade carbon fibers. More particularly, the present invention relates to an improved process for preparing high strength, high-modulus carbon fibers by subjecting a bundle of fibers of a carbon fiber precursor to infusibilization (or thermosetting) and carbonization treatments.
(2) Description of the Related Art
Primarily, carbon fibers were prepared from rayon as the starting material. Then various starting materials and preparation processes were investigated and developed. At present, however, in view of the properties of the products and also from an economical viewpoint, carbon fibers are substantially limited to polyacrylonitrile (PAN) type carbon fibers prepared from polyacrylonitrile and pitch type carbon fibers prepared from a coal or petroleum pitch.
Usually, pitch type carbon fibers have a low strength or modulus, and therefore, they are used as general-purpose grade carbon fibers in the fields of construction materials, heat-insulating materials, and packings. In contrast, PAN type carbon fibers have a high strength and a high modulus, and therefore, most high-performance grade carbon fibers are PAN type carbon fibers.
Recently, however, increasing interest has been shown in the technique of preparing high-performance grade carbon fibers from pitch For example, there have been proposed (a) a process in which pitch fibers obtained by melt-spinning a meso-phase pitch are subjected to an infusibilization treatment in an oxidative atmosphere at an elevated temperature and the fibers are then subjected to a carbonization treatment in an inert gas atmosphere at a higher temperature to obtain high-strength high-modulus carbon fibers (see Japanese Unexamined Patent Publication (Kokai) No. 49-19127, No. 53-65425 and No. 53-119326), (b) a process in which neomeso-phase pitch or dormant meso-phase pitch that can be easily converted to a meso-phase at the spinning step is spun and subjected to infusibilization and carbonization treatments (see v 15 Japanese Unexamined Patent Publication (Kokai) No. 55-58287 and No. 57-100186), and (c) a process in which a premeso-phase pitch that is converted to a meso-phase at the carbonization step is used (see Japanese Unexamined Patent Publication (Kokai) No. 58-18421).
In each of the foregoing processes for preparing high-performance grade carbon fibers, defects caused during the infusibilization treatment result in a drastic degradation of the strength in carbon fibers.
More specifically, at the infusibilization treatment step, various reactions are caused by heating precursor fibers in an oxidative atmosphere at a relatively high temperature close to the melting point or decomposition point of the fibers, whereby the fibers are converted to fibers having no melting point and capable of resisting the subsequent carbonization treatment. Where a multifilament bundle has poor separability, fibers are fused or softened before the fibers are rendered infusible, and fusion bonding occurs among the fibers. The multifilament bundle is hardened and made brittle by this fusion bonding, and thus the formation of fluffs or yarn breakage occurs at the carbonization step and the strength and elongation of the bundle as a whole are reduced Furthermore, surface defects are formed in the fusion-bonded portions of single fibers, resulting in a reduction of the strength, and these defects inhibit any manifestation of a high strength.
As means for eliminating this fusion bonding during the infusibilization treatment, there have been proposed and practiced (a) a method in which coal dust is sprayed onto the filaments before the infusibilization treatment (USSR Patent No. 168,848), (b) a method in which an active carbon powder impregnated with an oxidant is sprayed onto the filaments (U.S. Pat. No. 3,997,645), and (c) a method in which pitch fibers are treated in a dispersion of graphite or carbon black in water containing a water-soluble oxidant and a water-soluble surface active agent dissolved therein (U.S. Pat. Nos. 4,275,051 and 4,276,278).
In each of these methods, a good lubricating property is produced among the precursor fibers by a carbonaceous fine powder; preventing physical damage to the surfaces of fibers during subsequent treatments and any accumulation of heat at the time of oxidation.
To solve the problem of fusion bonding among fibers during the infusibilization treatment step for forming high-performance grade carbon fibers, the present inventors actually carried out the foregoing methods. As a result, it was found that although the carbonaceous fine powder has an effect of preventing fusion bonding during the infusibilization treatment step, it has a negative effect on the strength and elongation of carbon fibers obtained by the sintering treatment. More specifically, in each of the foregoing methods, fusion bonding among the fibers is prevented, but the strength and elongation of the resulting carbon fibers are lower than those of carbon fibers obtained by performing the infusibilization and carbonization treatments without the application of the fine powder. It is considered that this undesirable phenomenon occurs because the carbonaceous fine powder per se generates defects on the surfaces of fibers during the infusibilization and carbonization steps for some as yet unknown reason.
This phenomenon is observed not only when high-performance grade carbon fibers are prepared from pitch fibers but also when high-performance grade carbon fibers are prepared from polyacrylonitrile fibers.